Shaky video modes

[Ummon]

I did some searching and couldn't find anything on this. I'm using a JPAC with my candy, the monitor is standard res, and some modes will have a slight vertical shake up and down Like, someone's jiggling the picture every couple seconds. It was intermittent, and was on certain modes, but now it's essentially all the time on any mode below 368x240. Andy said it shouldn't be the JPAC, and suggested to adjust the monitor. I checked this, and it just rolled if the Vhold was adjusted, so I'm guessing that's where it's supposed to be. The effect is independent of hardware - except when using the tetris board that came with it, in which case it doesn't happen. Does anyone have any ideas?

[Ummon]

I'd like to reiterate that the Tetris board (game resolution is 336x240) is stable, yet when I run 336x240 from my PC, the image shakes. Anything above that is stable. I'm using soft15khz, incidentally, but remind that the shaking phenomenon has increased in frequency and scope.

[ken_cinder]

Sync wiring? Some monitors only like composite sync on the Horizontal pin, could be exacerbated by certain resolutions.
I'd try playing with that, you might also look to the B+ if adjustable, they are in the 29" Nanao's I have.
I had a waviness problem on a Hantarex monitor because of that, was like a quick vertical "wiggle" across the screen edges.

[Ummon]

I'll look again, but I couldn't find anything that looked like a pot of any sort the other night, though that may be because there's not really any room to see through the back panel hole of the candy, and that's the only port.

[Ummon]

I actually found some little plastic pot off yonder but turning it all the way either direction made no difference in any manner in the image.